The present invention relates to a by-pass valve apparatus of an exhaust turbocharger for an internal combustion engine.
One of the known by-pass valve apparatus of a turbocharger for an internal combustion engines is shown in FIG. 1. An exhaust conduit 2 of an internal combustion engine 1 includes a turbine 3a of a turbocharger 3. The turbine 3a is driven by the exhaust gas flow in the exhaust conduit 2 and drives a compressor 3b which is connected with the turbine 3a. The compressor 3b is inserted in a suction conduit 5 and accelerates the suction air flow in the suction conduit 5 which supplies the suction air to the engine 1 through a throttle valve 5a which regulates the suction flow. Fuel injection valves 1' supply fuel to the suction side of the engine. The exhaust conduit 2 has a by-pass passage 6 which by-passes the turbine 3a. A by-pass valve apparatus 7 includes a valve stem 7a carrying on its one end a valve element 7b which regulates the gas flow in the by-pass passage 6, and a diaphragm device 8 having a diaphragm 9 which, in turn, is connected with the valve stem 7a. The diaphragm device 8 has a pressure receiving chamber 8a which is connected through a pressure conduit 10 with the suction conduit 5 at the outlet of the impeller of the compressor 3b, an air chamber 8c which is separated from the pressure receiving chamber 8a by the diaphragm 9 and is connected with atmosphere through an orifice 8b, and a spring 11 which is mounted in the air chamber 8c and urges the diaphragm 9.
Static pressure P.sub.2 at the outlet of the compressor 3b is transmitted through the conduit 10 into the pressure receiving chamber 8a and urges the diaphragm 9 against the spring 11 to control the opening of the by-pass valve element 7b through the valve stem 7a. Thus, the exhaust flow which drives the turbine 3a is regulated so as to control the driving force of the compressor 3b. FIG. 2 shows detail of conventional construction of the pressure conduit 10 which is connected with the suction conduit 5 downstream of the outlet of the impeller of the compressor 3b to receive the static pressure P.sub.2.
In the construction of the known by-pass valve apparatus referred to above, the set value of the static pressure P.sub.2 at which the by-pass valve element 7b opens is determined with regard to knocking and damage of the engine 1. More specifically, when the by-pass valve element 7b is opened at the full open condition of the throttle valve 5b, the static pressure P.sub.2 is maintained at the set value even when the rotational speed of the engine 1 is increased, to prevent damage to the engine at the maximum rotational range during which the engine is subjected to the severest mechanical stress. Since the torque of the internal combustion engine is proportional to the boosted pressure, and since the boosted pressure is proportional to the compressor pressure and to the static pressure P.sub.2, maintenance of the static pressure P.sub.2 at the set value will result in maintaining the engine torque at a constant value as shown by the dash-dot line in FIG. 5.
As described, the known by-pass valve performs a suitable control at the maximum rotational range of the engine. However, at the intermediate and low speed ranges of the engine also, the by-pass valve is opened to maintain the engine torque constant, regardless of margins for the engine rotation and the by-pass valve opening, so that torque properties at the intermediate and low speed ranges of the engine cannot be fully utilized.
In Diesel engines and petroleum engines for automobiles, it is desirable to improve not only the maximum output but also the torque properties at the intermediate and low speed ranges.
To improve the torque properties at the intermediate and low speed ranges, it is necessary to increase the rotational speed of the turbocharger as high as possible. Although the by-pass valve may be controlled by detecting rotational speed of the turbocharger, the construction will be complicated. The static pressure P.sub.2 changes by suction air flow rate, even though the rotation of the turbocharger is kept constant. Thus, conventional turbocharger control on the basis of static pressure P.sub.2 is not sufficient to cover the whole speed range of the engine.